There are several specific areas which we will pursue during the next 3 years of this project. They are: 1. Comparison of peptide maps of platelet actin I and actin II. These studies will be coupled with isoelectric focusing studies on the two forms of actin. 2. Isolation and further characterization of the actin binding protein - actin complex. Purification of the actin binding protein has now been achieved and its effect on actin polymerization is under investigation. The role of divalent cations in the interaction of ABP-actin will also be investigated. 3. A contractile apparatus from platelets, similar to the muscle sarcomere and containing, ABP, actin, myosin and alpha-actinin can now be isolated. The localization of these proteins and their interactions in the apparatus will be investigated. 4. In conjunction with (3) above, the localization and form of the platelet contractile proteins will be investigated before and after stimulation with agents, such as thrombin, collagen, ionophore and ADP. This model system may be as useful as the glycerinated muscle fiber in providing useful information concerning the role of contractile proteins in platelet function. 5. Continuation of studies on the structure and role of the calcium binding protein present in platelets.